95 GHz polarimetric radar observations of ice crystal aggregates

Millimeter wave radars operating at 95 GHz are now being used for the remote sensing of ice clouds from ground-based and airborne platforms. Two important goals are to identify the crystal types and to estimate bulk parameters such as the ice water content. Polarimetric radars have the potential for doing both. Two-dimensional particle probe (2D-C cloud and 2D-P precipitation probe) measurements in clouds show that ice crystal aggregates are observed more often than "pristine" or single type of ice crystals (e. g., column, plate, stellar crystal, etc.). This paper focuses on polarimetric radar signatures of ice crystal aggregates and mixtures with pristine crystals. Measurements with the University of Wyoming's airborne 95 GHz radar system and particle probe measurements are used in this study. Data from field experiments conducted in 1997 (February through April) are analyzed. During these experiments as the aircraft (King Air) penetrated through the clouds, the onboard radar and particle probes simultaneously collected data. Polarimetric radar signatures of ice crystal aggregates were obtained with the assumption that the closest four range gates (120 to 210 m, with 30 m gate spacing) from the radar were highly correlated with the images obtained by probes on the aircraft wing. As expected, the aggregates observed during these experiments showed clustering characteristics on the Zh - ZDP, Zh - ZDR, and Zh - LDRv planes that were quite different than planar (plates and stellar crystals) and columnar crystals. However, these characteristics could not be explained based on models with a spherical shape or any other shape with random orientation. The aggregates displayed radar signatures indicating some orientation with their larger dimensions along the horizontal direction. Mixtures of aggregates and pristine crystals showed distinct signatures as well.